SPE 124689
Tracer Detection by Laser Spectroscopy for Applications in the Oil and Gas
Industry
R. Nava, Texas A&M University, H. Schuessler, M. Fahes and H. Nasrabadi, Texas A&M University at Qatar, and
A. Kolomenski, Texas A&M University
Copyright 2009, Society of Petroleum Engineers
This paper was prepared for presentation at the 2009 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, USA, 4–7 October 2009.
This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been
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Abstract
Tracer technology is very popular in petroleum engineering applications, and includes applications in reservoir
characterization, reservoir modeling, improved oil recovery, etc. Most of the current tracer technology uses radioactive
isotopes that are not environmentally friendly and require strict safety precautions. We propose the use of rare long-lived and
stable noble gas isotopes as tracers, where instead of the decay of the radioactive material, we will use optical detection.
Due to their chemical inertness, rare noble gases offer the advantage that they do not react with the environment with which
they are in contact. Our research employs for the first time optical detection of tracers by collinear fast beam laser
spectroscopy. It has the additional advantage that novel tracer projects based on multiple tracers can be designed in the future.
In this work also the optical hyperfine structure of long lived 85 Kr was used to identify the 85 Kr tracer atoms. The technique
has also been successfully applied to stable krypton isotopes. The abundance selectivity is at the one part in 1010 level and the
sensitivity is at a few hundred ions. This method is several orders of magnitude more sensitive than the standard nuclear
decay detection.
The new tracer technology offers a safer and more accurate option for applications in the oil and gas industry. The research
work is currently being done in Qatar for application in Qatar’s North Field.
Introduction
Tracer technology has very important applications in the oil and gas industry. Chemical or radioactive tracers are used to
label fluids from specific locations in the reservoir, and trace them as they appear at other locations (Huseby et al., 2008). The
results are used to evaluate inter-well connectivity and reservoir continuity, detect high permeability zones (Anisimov et al.,
2009), identify residual oil saturation during waterflooding or miscible gas flooding applications (Devegowda et. al., 2009;
Cockin et al., 2000), assess the success of fracture treatments (McDaniel et al., 2009), and many other applications such as
precipitation in the near-wellbore region, and reservoir fluid sampling (Stamatakis et al, 2006; Gozalpour et al., 2005).
The application of tracers in the petroleum industry has been around for decades (Gore and Terry, 1956; Watkins and
Mardock, 1954), and has proven to yield more accurate results on reservoir characterization than the use of pressure and rate
data (Datta-Gupta et al., 1997; Huseby et al., 2009). Radioactive tracers such as Radon or natural tracers such as the ioncontent of water are used to track injected fluids and identify the paths they traverse. However in most of the applications, the
results yield qualitative evaluations rather than quantitative information about the reservoir.
The new tracer technology, which we are proposing for application in oil and gas reservoirs, has the advantage of a very high
selectivity and sensitivity over traditional tracer technologies. The use of optical detection rather than radioactive decay or
mass spectroscopy results in the ability of detecting about one tracer atom of 85 Kr in 1010 atoms in the natural background of
84
Kr . This is achieved using collinear fast ion beam laser spectroscopy detection. Rare noble gas isotopes are ideal
candidates for such an application. In the next sections we review the technology of optical detection, discuss the
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experimental setup and procedure for our work, and present some of the problems in the petroleum industry that can be
addressed by applying this technology.
Theoretical Background
The collinear fast beam laser spectroscopy (CFBLS), introduced first by Kaufman (1976) and Wing et al. (1976) has evolved
into a powerful tool of ultra-sensitive rare isotope detection (Iimura et al., 2003). It offers high sensitivity and selectivity due
to several main features. First, mass separation of isotopes can be easily incorporated. Secondly, Doppler broadenings of the
spectral lines can practically be eliminated by velocity bunching. An artificial isotope shift introduced in CFBLS by
accelerating the ions to a large energy further separates the spectral lines of different isotopes. Finally, the ions are converted
into neutral atoms by charge exchange collision readily allowing for the optical excitation of an atom beam in and around the
visible region, which is much easier implemented, than the excitation necessary for ion beams requiring the UV spectral
range.
The present work on krypton was motivated by the fact that 85 Kr is a major tracer gas for exploring the reservoir structure of
large oil fields. 85 Kr detection in ambient air is also of importance for monitoring nuclear activities on a world wide scale
(Otten, 1989). Our particle detection version of CFBLS combines small sample size with a high potential to be both an ultrasensitive and ultra-selective detection scheme for 85 Kr . We obtain a high selectivity by sequential two step excitation with
CW narrow band lasers to a suitably chosen Rydberg level, followed by field ionization. Figure 1 displays the relevant
energy levels of Kr for this scheme. In such a technique only signal ions, which have been resonant with two laser
frequencies, are field ionized and detected. Such particle detection has obvious advantages over photon detection: the events
can be detected with close to unity efficiency and the background of the scattered laser hght is eliminated. In addition, we use
energy discrimination in an energy filter. This is possible, since the signal ions are produced with a characteristic energy,
which is selectable by the field ionizer setting. The background ions produced as a result of collisions are to a large extent
eliminated by the energy filter. Because of these advantages CFBLS has been the major technique for on-line and off-line
laser spectroscopy of short-lived isotopes (Otten, 1989) as well as in trace isotope detection in samples taken from the
environment, most notably for monitoring long-lived
produced in nuclear fall out (Wendt et al., 1997).
Our theoretical analysis for the stepwise excitation is approximated by the three level atom model and summarized in
Figures 2 and 3.
Experimental Setup and Procedure
The experimental setup is depicted in Figure 4. A hollow cathode discharge ion source (Danfysik 911A) generates a beam of
Kr ions. The ions were extracted and accelerated to between 5 and 12 keV and mass-separated with a high transmission lowThe mass-selected ion beam is then deflected by a few degrees and co-propagates
resolution bending magnet
with laser beams from a Ti: Sapphire ring laser and a single-mode
laser. The beam-line is evacuated by turbomolecular
pumps to pressures below
torr. After passing through a post acceleration stage, the ion beam enters an alkali charge
exchange cell for neutrahzation. In the near-resonant charge exchange reaction either cesium or rubidium vapors are used. A
dominant fraction of the ions is thereby transferred into the metastable
and
levels. The remaining ions
are deflected and the fast neutral metastable atoms enter the interaction region of about 0.8 meters in length. Thereafter the
atoms pass through a collisional ionization cell or a field ionizer. In this way the Rydberg atoms can be either collisionally
ionized or field ionized. The signal ions, after passing through a 135° energy filter, are detected with a Faraday cup or for
low-level counting with a channeltron.
(Coherent Innova 200) is used.
For the first excitation step a Ti: Sapphire ring laser (Coherent 899-21) pumped by an
The linewidth of the actively-stabilized Ti: Sapphire laser is around 3 MHz. For the second-step excitation a single-frequency
laser (Coherent CR-2000K, operated at 488 nm wavelength) and equipped with a 1.0 cm fused silica solid state etalon is
employed. This laser has a similar line-width of a few MHz. We monitor the single mode structure with a Burleigh 1.5 GHz
scanning Fabry-Perot. A Burleigh wavemeter (resolution 0.01 cm-1) helps to coarsely adjust the frequencies of the Ti:
Sapphire laser. The two laser beams were combined using dichroic mirrors and directed collinearly into the fast beam
apparatus.
The collisional ionization cell is made of a small gas chamber containing the collision gas. It has two apertures on either side
of the walls. The chamber is encased in an outer cylindrical cell, which is differentially pumped by a turbomolecular pump
(Leybold turbovac 361). The dependence of the collisional ionization cross-section with respect to different target gases has
been investigated previously (Neugart et al., 1986; Borchers et al., 1989). For our collisional ionization cell with an effective
cell length of 50 mm the best signals were obtained for collisional target gas pressures ranging from 40 mTorr to 100 mTorr.
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. Following the
Specifically, our scheme involves the following cascade excitations:
excitation, which is optimized by the laser power, atoms in the
level are field ionized. With the ion counting of
the hyperfine structure resonances of 85 Kr , providing a unique fingerprint, the minute quantities of this isotope are detected.
Collisional ionization detection proved very helpful in tuning and maximizing the two sequential laser excitation steps. It was
important to tune the recyclable transition transition at 811.5nm. Figure 5-a shows the “flop in” signal observed in this case.
For other not interesting first excitation steps “flop out” signals like depicted in Figure 5-b were recorded. In such a case the
atoms are lost, since they can decay back to the ground state.
An important prerequisite for reaching high detection sensitivity was to optimize the charge exchange ratio. Figure 6
demonstrates that when using Rubidium in the charge exchange cell 80% neutralization yields about 80% detection
efficiency (8 on a scale of 10 in Figure 6).
For the initial observation of the hfs splittings of the various transitions a natural krypton sample slightly enriched with
85
Kr was prepared. The 85 Kr concentration was about 10-6. With it the first excitation step from the
was readily detected using collisional ionization only. It is noted that such a signal can only be observed with an enriched
sample, since otherwise the resolution of the one-step excitation is not sufficient to suppress the background from the
neighboring 85 Kr isotope even though mass separation was used. After the hfs of 85 Kr had been determined in this way, we
employed a more dilute sample produced at the TAMU reactor with a 85 Kr concentration of 10-8. The 85 Kr concentration for
this sample and subsequent ones obtained by dilution were determined by low level β-counting, and alternatively also from
the measured ratio of the 85 Kr to 84 Kr signals observed with the complete cascade two-step excitation field ionization
method. Figure 7 compiles 85 Kr concentration measurement from various samples. Typical results for the recyclable hfs
transition of 85 Kr with different sample concentrations show reasonable agreement, with the CFBLS optical measurement
being consistently lower by about a factor of two. Further dilutions established our presently best value for the selectively of
and the sensitivity of a few hundred 85 Kr atoms/s.
about
Proposed Applications in the Oil and Gas Industry
Qatar’s North field is the largest known non-associated gas field in the world (approximately 20% of the world’s known gas
reserves). The reservoir consists of four producing layers from the Khuff formation (K1, K2, K3 and K4). The field produces
from a carbonate gas-condensate reservoir with a Condensate-Gas-Ratio (CGR) of about 40 STB/MMSCF. The reservoir
initial pressure (5315 psi) is close to the dew point pressure (5135 psi) indicating that condensate dropout occurs since the
reservoir is being produced below dew point (Whitson et al., 2005). The proposed tracer technology can be applied to tackle
many of the problems and answer some of the questions that the petroleum industry in Qatar faces. Some of these problems
are the following:
1.
2.
3.
4.
Calculating the contribution of each layer in producing commingled wells. This can be achieved by analyzing fluid
samples from each layer and identifying natural tracers or tracers injected into these layers at other well locations.
The results can also be used to detect problems that might be associated with formation damage in some of the
layers. This may include the analysis of condensate dropout.
Studying the connectivity of various blocks. There are indications of compositional variation in the North Field, and
although this can be the result of different source rocks, biodegradation, diffusion or other physical phenomena, it
could also be due to low-permeability barriers. Studying the connectivity of various wells, the locations of barriers
and of high permeability streaks is very important for better development of this important hydrocarbon resource.
Detecting leaks from the gas fields by analyzing air samples. Measuring tracer gas content in air samples taken from
various locations can lead to a better understanding of the reservoir in terms of its sealing capability. The high
selectivity and sensitivity of this method makes it a great candidate for such an application.
Improving fluid sampling procedures. When tracers are added to the drilling fluid, it will be easier to detect any
infiltration to the fluid sample.
Although current tracer technology could address these issues, the proposed technology results in much more accurate results
and is surely safer than the application of radioactive tracers. Our work will continue towards the application of this
technology in Qatar. We are in the process instituting a tracer laboratory by putting together this experimental setup in Qatar
and acquiring gas samples from Qatar’s North Field for testing.
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Conclusions
In this paper, we present a summary of the work that was conducted in order to develop the application of collinear fast ion
beam laser technology in Qatar. Current tracer applications in the oil and gas industry will benefit from added accuracy ,
safety, and simultaneousmultipletracer methods associated with this new technology. The method of optical detection
develloped here is orders of magnitude more accurate than conventional tracer detection methods. It has abundance
selectivity at the one part in 1010 level at mass 85amu and the sensitivity is at a few hundred ions.
Acknowledgements
This research is supported by the Welch Foundation, grant A-1546, the National Science Foundation under grant No.0722800
and the Qatar National Research Fund, grant NPRP30-6-7-35.
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Figures
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Figure 1 – The excitation scheme (Beam energy 5-12ev)
Figure 2 – Level population and atomic coherences for a three-level atom interacting with two laser fields.
Figure 3 – Calculated population dynamics at different Ar+ powers
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Figure 4 – Experimental Setup
Figure 5 – Detection of Kr optical transitions by collisional ionization.
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Figure 6 – Detection efficiency vs. neutralization ratio and charge exchange media.
Figure 7 – Results of concentration measurements in various samples.